Low temperature firing multilayer ceramic circuit boards are known that are suitable for use with low melt temperature conductive metals, such as silver, gold and copper. They have a low thermal coefficient of expansion (TCE) and they may be made to be compatible with both silicon and gallium arsenide devices. These ceramic circuit boards are made from glasses that can be fired at temperatures of less than 1000.degree. C. They are made by admixing finely divided selected glass particles or powders with organic materials, such as resin, solvents, dispersants and the like, and then the resultant slurry is cast as a thin tape, called green tape. A circuit pattern may be screen printed onto the green tape using a conductive ink formulation comprising a conductive metal powder, an organic vehicle and a powdered glass, generally the same as, or a similar glass, to that used to make the green tape.
When a plurality of green tapes are used, via holes are punched into the tapes, which vias are then filled with a conductive via fill ink, made with a conductive powder, an organic vehicle and a suitable glass, to provide electrical contact between the circuits on adjacent green tape layers. When all of the desired green tapes have been patterned, they are aligned and laminated under heat and pressure prior to firing.
More recently, the multilayer ceramic circuit boards have been adhered to a metal support substrate which increases the strength of the multilayer board. When a bonding glass is used to adhere the green tapes to the support substrate, an additional advantage is obtained because the bonding glass reduces the shrinkage of the green tapes during firing in the x and y dimensions, so that most of the shrinkage occurs only in the z, or thickness, dimension. This means the printed circuits can be made with closer tolerances. The glasses used in the green tapes however, must have a TCE matched to that of the metal support to prevent delamination or cracking of the fired glass. Mixtures of crystallizable and non-crystallizable glasses can be used, and inorganic fillers can also be added so that the TCE of the green tape glasses match that of the metal support.
Passive components such as resistors and capacitors can also be embedded in a green tape stack. Suitable resistor or capacitor inks can be screen printed onto green tapes to obtain tight tolerances and high precision placement of the passive components.
Screen printed capacitors are known based on barium titanate and lead magnesium niobate dielectrics. The selected dielectric is admixed with suitable glasses and an organic vehicle so as to obtain a capacitor ink which can have a wide range of dielectric constant. The capacitor ink is screen printed onto a green tape. The screen printed capacitor layers are connected to a silver conductor layer screen printed on green tape layers over and under the capacitor printed layer by means of vias in the green tape that are then filled with appropriate conductor via fill inks.
The above-described screen printed embedded capacitors, while they are an important advance in the art, have been limited in their dielectric constant and are limited in their size; which is dependent on the size of the chip to which they are used. In general, these embedded capacitors have a maximum practicable size of about 6.times.6 mm. Up till now, the highest dielectric constants achieved reproducibly for these buried capacitors has been about 1600. Thus a search has continued for screen printable capacitor inks and for capacitor structures having a high dielectric constant that can be made reproducibly.